The day after the events in Friday 13th Part II, Jason, seriously injured after his encounter, goes to a lakefront store to find clothes and a replacement mask. While there he kills owners Harold (meat cleaver in the chest) and Edna (stabbed in the back of the head with a knitting needle) before moving on to Higgin’s Haven, a local farmhouse. At the same time, Chris Higgins and her friends return to Higgin’s Haven to spend the weekend. The gang includes pregnant Debbie, her boyfriend Andy, prankster Shelley, his love interest Vera (who initially does not reciprocate his feelings), stoners Chuck and Chili, and Chris’s boyfriend Rick.
Shelley and Vera inadvertently run afoul of bikers Ali, Loco and Fox at a convenience store. After being wronged, Shelley stands up for himself by running over their motorcycles, impressing Vera. Rick loses his temper after seeing the window of his Volkswagen Beetle smashed in and leaves with Chris. Meanwhile, Jason has hidden in the barn to recover, and when the bikers show up to siphon their gas with the intention of burning down the barn to get even, he kills Loco and Fox with pitchforks and bludgeons Ali into unconsciousness. As night falls, Jason dons a hockey mask (FINALLY!) he stole from Shelley after a prank he pulled on Vera and shoots her through the eye with a speargun. He moves into the house where he kills Andy by bisecting him as he walks on his hands, then Debbie by stabbing her through her neck in the hammock she is lying on. He turns off the power in the house, prompting Chili to send Chuck down to check on the power. Jason throws Chuck into the electric box and kills him, severing power to the house. Shelley appears to Chili with a slashed neck, but she ignores him, thinking he’s pulling a prank and he dies. She discovers Shelley is dead, then discovers her friends’ bodies upstairs. As she tries to escape, Jason kills her by impaling her with a fireplace poker.
Chris tells Rick about a time she was attacked by a horrible, disfigured man two years earlier, which was the reason she moved away. Rick’s car dies and they have to walk back to the Haven, which they find in disarray. Rick steps outside to search the grounds, but Jason grabs him just beside the cabin. As Chris calls out to him from the front door, Jason holds him back just out of sight and keeps one hand held over his mouth. With no response from Rick, Chris goes back inside, literally leaving Rick’s life in Jason’s hands as he crushes Rick’s skull with them. Chris discovers bloody clothes in the overflowing tub upstairs. She runs outside and is surprised by Rick and Loco’s corpses dropping down from a rope on a tree limb. Chris runs back inside, and Jason throws Rick’s corpse through the window. Terrorized and panicked, she fends Jason off and hides upstairs in a hall closet. She discovers Debbie’s body, and narrowly escapes the house and tries to escape in her van, which breaks down after having been siphoned by Ali and the others. She makes her way to the barn where she tries to hide and is attacked again by Jason. She lures Jason up to the loft and sends him through the window, hanging him, but he breaks free, much to Chris’ horror. Ali recovers and attacks him, but he is brutally butchered as Chris finds a felling axe and strikes Jason in the head with it, and he collapses. Exhausted, Chris pushes a canoe out into the lake and falls asleep, waking in fright the next morning. She sees Jason in the barn and tries to escape as he comes after her, but it all turns out to be a hallucination. As she calms down, the decomposing body of Pamela Voorhees emerges from the lake and grabs her, dragging her into the water, which turns out to be another hallucination.
Later, police are escorting a clearly disturbed and hysterical Chris from Higgins Haven, stating that she was deeply shaken by the deaths of her friends. As she is driven away, Jason’s body is shown lying in the barn and the lake seems to be once again at peace.